We intended to show the changes in composition and membrane structure of salted Alaska Pollack roe (“Tarako”) and its red pepper seasoned product (“Karashi Mentaiko”) during the manufacturing process, and to propose a new method for the evaluation of fine structure of the roe with a polarization microscope.
The samples were collected from each stage of the salted process and the seasoned process. The chemical compositions of the roe and its seasoning liquid, the shapes and the rheological properties of the ovary and the egg were investigated. The fine structure of the egg was observed under the crossed nicols with a polarization microscope in addition to the sensory evaluation.
The components of the roe were classified into two groups judging from the change of the composition during the salted process: one with a steep change in a concentration in the early stage and then a succesively slow change to an asymptotic value at the final stage such as moisture and ash, and another with a gradual decrease in the all stages such as protein, lipid (fatty acids) and carbohydrate. The increase in the salt concentration which was main component of the ash was conspicuous and was considered to be due to the decrease in the viscosity of the medium of the ovary. This decrease was caused by rotating the vessel containing the ovary with high concentration of salt under an elevating temperature resulting in the accelaration of diffusional rate of NaCl molecules. The concentration of moisture and ash changed obviously during the seasoned process and the one of the salt reached to the equilibrium level at 4 days after starting.
The egg was deformed into the spherical shape, and the elasticity of the roe increased during the salted process. The volume of the medium in the ovary increased slightly and the viscosity decreased remarkably during the seasoned process. The molecular orientation in the egg evaluated through the polarization microscopy, developed rapidly in the initial stage and leveled off at the end of the salted process. Even when the egg was damaged by a physical accident, the orientation was not disturbed in other part than the damaged point. These results indicate that (1) NaCl molecules which diffused into the roe gave rise to the irreversible changes in its membrane structure such as its molecular orientation, and (2) this structural change made the elasticity of the membrane increase and gave the membrane semipermeability.
